textproduct: Albany

WHAT HAS CHANGED

Given increasing consensus on accumulating wet snow in higher terrain areas (especially at and above 1500 feet) Thursday afternoon through Friday morning, winter weather advisories were issued for Hamilton, northern Warren, western Greene and western Ulster County in New York and Bennington and western Windham County in Vermont.

There is also increasing confidence that even valley areas from the Capital District northward will observe a changeover to wet snow for a period Thursday night into early Friday. If temperatures become cool to support light accumulations, most will be limited to cooler/grassy surfaces.

KEY MESSAGES

1) An early season accumulation of wet snow is likely across higher terrain areas Thursday into Friday thanks to a storm system passing to the south. There remains some uncertainty if temperatures will become cool enough in valley areas to support light snow accumulations, mainly on cooler/grassy surfaces.

DISCUSSION

KEY MESSAGE 1...

All eyes remain focused on a developing coastal low that will likely usher in our first accumulating wet snow of the season tomorrow into Friday. Latest water vapor imagery shows a potent shortwave tracking through the Midwest and rounding the base of broad troughing developing over southern Canada. Heights fall ahead of the shortwave and developing sfc low as it continues to track into the Ohio Valley and mid-Atlantic tonight into early tomorrow. Increasing forcing for ascent and strengthening warm air and moisture advection looks to quickly expand into the Northeast including eastern NY and western New England by 06 to 12 UTC tonight allowing mainly rain to begin. The higher terrain of the western Adirondacks should be cool enough that p-type favors rain/snow mix initially before becoming mainly wet snow. Steady rain continues across the region, especially along and north of I-90 where the stronger warm air/moisture advection resides, through tomorrow morning making for a wet Thursday morning commute. The ongoing rain will keep temperatures cool from wet-bulinb/diabatic cooling effects with most staying in the 30s and 40s through the day. The southern Adirondacks will remain wet snow through the day with even the foothills becoming a rain/snow mix given cooling temperatures. Elevations at and above 1500ft in southern VT and the northern/eastern Catskills will eventually succumb to the cooling temperatures with rain becoming a rain/snow mix by the afternoon and the spine of the Greens becoming mainly wet snow.

Our sfc low deepens further as it reaches the mid-Atlantic/southern New England coast and undergoes cyclogenesis by Friday evening as the shortwave takes on a negative tilt. North-northeast winds on the north side of the low will support cool air advection into eastern NY and western New England Friday night. Mid-level frontogenesis looks to support mesoscale banding on the northwest side of the storm which should support periods of moderate precipitation overnight. There remains uncertainty regarding the exact storm track which will be important in the placement of the developing mesoscale bands. Latest trends track the low a bit further south which would place the bands mainly from the eastern Catskills north and east into southern VT but this is only a low to moderate confidence in the exact placement. Where these bands form will have higher potential for heavy enough precipitation to occur that diabatic cooling effects can allow temperatures to cool towards their respective wet-bulb temperatures and support a changeover to wet snow. Latest forecast soundings continue to show cool air advection from breezy northerly winds combining with diabatic cooling effects that thermal profiles even in valley areas from the Capital District northward support a changeover to wet- snow mainly 06 - 12 UTC Friday. While surface temperatures will be marginal only in the low to mid-30s, where any mesoscale bands that can result in moderate/heavy precipitation rates will have the highest chance for wet snow to accumulate. Given rather warm ground surfaces and a lack of an antecedent cool air mass, any wet snow accumulations in the valley will likely be limited to grassy/cooler sfcs with roads likely remaining wet. Our latest forecast continues to show a few tenths of snow in the valley to help message this potential with NBM probabilistic guidance showing a 25 to 40% chance of at least 0.1" of snow through early Friday morning.

Higher terrain areas including the southern Greens, northern/eastern Catskills, Helderbergs, western MA, Taconics, western/southern Adirondacks have the highest chance for accumulating wet snow with the latest NBM probabilistic guidance showing a 40 to 80% chance for at least 2 inches of snow and a 30 to 60% chance for at least 4 inches in the 24 hr period ending 7am Friday. The latest probabilistic WSSI values have also trended upwards for "minor impacts" with values now ranging 20 to 40% with even isolated pockets of 50% in the southern Greens/eastern Catskills. Given these trends and slight increase in wet snowfall amounts, we issued the first Winter Weather Advisories of the season for the southern Adirondacks, southern Greens, and eastern Catskill from Thursday into Friday morning where amounts range 2 to 5 inches. While these amounts are on the low end of our advisory criteria, we erred on the side of caution and went with the advisory given wet snow character that can lead to isolated power outages, this being the first event of the season and potential slippery travel for the Friday morning commute. We also show 1 to 3 inches of wet snow in the Helderbergs, higher terrain of western MA into the Rensselaer Plateau. The recent southerly shift in the storm track would favor any mesoscale banding placement in the eastern Catskills with upsloping potentially leading to locally higher amounts but again there is some uncertainty in the exact storm track.

Recent trends show the precipitation rates turning lighter rather quickly Friday morning as the coastal low exits off the New England coast and northerly winds advect drier air into the region. While there are signs that an inverted trough may develop on the back side of low which would linger moist cyclonic flow and showers, the southerly storm track shift looks to place the better potential any impacts mainly to our south. Thus, we show a drying trend through the day with breezy northerly winds making for a chilly day as temperatures remain in the 30s throughout the region. Overnight lows Friday night drop into the 20s across eastern NY and western New England so cannot rule out some patchy slick spots given wet ground surfaces.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday, VFR conditions expected through at least midnight, despite thickening high/mid level clouds along with some patchy clouds with bases of 4000-6000 FT AGL. Light rain associated with approaching low pressure system will overspread the TAF sites between 08Z-12Z/Thu, starting at KGFL and KALB first. Cigs/Vsbys will gradually become MVFR after 1-2 hours of rain onset; with MVFR conditions (both Vsbys and Cigs) trending to IFR/LIFR between 15Z-18Z/Thu as light to moderate rainfall and some areas of fog develop.

West to northwest winds 5-10 KT this afternoon will become light/variable around or shortly after sunset, then will become northeast to north at 5-10 KT after daybreak on Thursday.

Outlook...

Thursday Night: High Operational Impact. Definite RA...SN. Friday: High Operational Impact. Likely SHRA...SHSN...SN. Friday Night: Low Operational Impact. Slight Chance of SN. Saturday: Low Operational Impact. Slight Chance of RA...SN. Saturday Night: No Operational Impact. NO SIG WX. Sunday: No Operational Impact. NO SIG WX. Sunday Night: No Operational Impact. NO SIG WX. Monday: No Operational Impact. NO SIG WX.

ALY WATCHES/WARNINGS/ADVISORIES

CT...None. NY...Winter Weather Advisory from 10 AM Thursday to 10 AM EST Friday for NYZ033-042. Winter Weather Advisory from 7 PM Thursday to 1 PM EST Friday for NYZ058-063. MA...None. VT...Winter Weather Advisory from 10 AM Thursday to 10 AM EST Friday for VTZ013-014.